Mastermind
Work Saw Specialist
It's just a common roloc mandrel with a longer shank added. The threads are 1/4 x 20.....
Ported 026 Squish Check
- Thread starter drf255
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I like the direction this thread is going..more of a cooperative R&D type thing. That's good.
Mastermind
Work Saw Specialist
If we all can check our egos at the door, we can have lots of threads like this.
We might never agree on what we think makes the "best" saw, but we can have respect for each other's ideas.
We might never agree on what we think makes the "best" saw, but we can have respect for each other's ideas.
Chris-PA
Where the Wild Things Are
This topic is mostly just simple geometry.
I looked at the last saw I worked on (my Earthquake G3800 clone), which has a simple cylinder/combustion chamber geometry (pictured). Roughly, the combustion chamber is hemispherical and half the diameter of the bore (38mm), which makes things really easy to calculate. The piston is actually domed, but ignore that for now.
The circumference at the edge of the combustion chamber is half that at the edge of the bore. Therefore at a minimum you'd need the squish distance to be twice as high at the edge of the combustion chamber as it is at the edge of the bore in order to avoid compressing the gas as it moves from the cylinder wall to the edge of the combustion chamber.
One of the main beneficial effects of cutting the squish band is actually just to reduce the combustion chamber volume and increase the compression ratio, but it is a rather inefficient way to do this. That's because the footprint of the combustion chamber is only ¼ of the area of the top of the piston. On this engine if you take the squish from 1mm (0.040”) to 0.5mm (0.020”) you reduce the volume of the effective combustion chamber by some 30%. If you then machine the squish band another 0.5mm while maintaining the 0.5mm squish you reduce the effective chamber volume by only 8%. And if you try to add a taper to the squish band you lose a lot of effective chamber volume because the area under squish band is so big.
However, that is only a consequence of using a flat piston top. If you have a pop up, or better yet had some way to add material to the actual combustion chamber, then you could increase compression ratio and maintain a tapered squish band. I realize there are concerns about pop ups interfering with gas flow from the transfers, but on the other hand I bet no one can find any research support for an untapered squish band either. So you have two things that are clearly not ideal, but the reality of designing things is that you are always trying to balance competing constraints and requirements.
I looked at the last saw I worked on (my Earthquake G3800 clone), which has a simple cylinder/combustion chamber geometry (pictured). Roughly, the combustion chamber is hemispherical and half the diameter of the bore (38mm), which makes things really easy to calculate. The piston is actually domed, but ignore that for now.
The circumference at the edge of the combustion chamber is half that at the edge of the bore. Therefore at a minimum you'd need the squish distance to be twice as high at the edge of the combustion chamber as it is at the edge of the bore in order to avoid compressing the gas as it moves from the cylinder wall to the edge of the combustion chamber.
One of the main beneficial effects of cutting the squish band is actually just to reduce the combustion chamber volume and increase the compression ratio, but it is a rather inefficient way to do this. That's because the footprint of the combustion chamber is only ¼ of the area of the top of the piston. On this engine if you take the squish from 1mm (0.040”) to 0.5mm (0.020”) you reduce the volume of the effective combustion chamber by some 30%. If you then machine the squish band another 0.5mm while maintaining the 0.5mm squish you reduce the effective chamber volume by only 8%. And if you try to add a taper to the squish band you lose a lot of effective chamber volume because the area under squish band is so big.
However, that is only a consequence of using a flat piston top. If you have a pop up, or better yet had some way to add material to the actual combustion chamber, then you could increase compression ratio and maintain a tapered squish band. I realize there are concerns about pop ups interfering with gas flow from the transfers, but on the other hand I bet no one can find any research support for an untapered squish band either. So you have two things that are clearly not ideal, but the reality of designing things is that you are always trying to balance competing constraints and requirements.
And a Merry Christmas to you too my friend!
Moparmyway
Its just a saw
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What if you tapered the squish band 3 degrees and cut the piston crown that same 3 degrees .............
then you cut 50% of the squish band, the 1/2 closest to the combustion chamber to say 30 degrees.
You would be increasing the size combustion chamber, reducing the larger 3 degree tapered squish band to 1/2 of its width, lowering some compression, although at readings of over 230 psi, one might reason that lowering compression a little might be beneficial
I am not talking a pop-up, but a 3 degre taper at the edge of the piston that matches the 3 degree taper in the outer squish band
Mastermind
Work Saw Specialist
Here's the jug in front of me right now. It's a closed port 268XP.
The bore is 50mm, and the squish band is 9mm wide.
Doesn't seem overly wide in my mind....
The bore is 50mm, and the squish band is 9mm wide.
Doesn't seem overly wide in my mind....
Mastermind
Work Saw Specialist
If you look closely you can see where the edge has been sanded.
Moparmyway
Its just a saw
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Just asking what if :
What if you kept that outer squish band to 5mm, and for the other 4mm closer to the combustion chamber, you tapered it by say 30 degrees ?
Mastermind
Work Saw Specialist
30° would be way too much. The goal is to increase squish velocity.
The reasons are increased power, and detonation resistance.
At 30° velocity would be lost.....
At least that's what I've been taught.
drf255
BAD CAD
On 4 stroke auto engines, the move has been towards larger squish/quench areas and smaller heart shaped combustion chambers, just enough to unshroud the valves.
More squish, more tumble, more complete charge combustion.
Why this wont translate to a 2 stroke, I'm not fully getting.
More squish, more tumble, more complete charge combustion.
Why this wont translate to a 2 stroke, I'm not fully getting.
Moparmyway
Its just a saw
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Sounds logical, if that chamber is stock.
If its been cut, then the squish band was widened, and that seems to be more beneficial to lower RPM machines from the technical information about combustion chamber squish theories.
If you open up the chamber some, after cutting the squish to a wider band, in my pea brain, you would have a larger area for the chamber, resulting in more initial hit after the plug is fired. You would still have a larger squish band area from machining it, yet you might be making up for area loss of chamber volume, spreading the chamber more across the piston crown ...................... no ?
Moparmyway
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Thats only in less expensive wedge heads, nobody will dispute that the hemispherical head rules in both efficiency and power production.
To splay the valvetrain for a hemi head requires additional cost and space
Stihlofadeal64
Addicted to ArboristSite
Speaking of R&D, should I be packing up my "guinea pig" saw for the UPS Santa Claus guys to bring over to your place after Christmas or will there be no peace on earth and "good will" to this onlooker
yeah I know, I'm having too much fun again ----> back to my corner
Stihlofadeal64
Addicted to ArboristSite
Chris-PA
Where the Wild Things Are
You need a taper relative to the surface of the piston, whatever that is, in order to control the gas velocity. If it's a domed or tapered piston then the squish band should not match that curvature. I have no idea what the right angles or widths are - that is part of what the software Brad linked to is for. However, in that description there was concern for the additional gas velocity that the pipe effects would bring, and of course here there is no pipe. So one again I'm really skeptical that rules of thumb for piped 2-strokes are directly applicable for chainsaws.
There is supposedly some gas velocity pushed out of the squish band that is adequate for proper mixing of the fuel/air in the combustion chamber, and going beyond that takes energy that is lost as heat.
From what I'm reading, the squishband taper is only 1°-2°. It's not a lot.
Mastermind
Work Saw Specialist
Without building a head, we are sorta stuck with the factory chamber. In many cases I want to get the exhaust port down, and that alone determines the amount I remove from the squish band.
Who here has put their eye on the new 661 combustion chamber, and measured the width and taper of the squish band?
I think if you take a hard look at what Stihl did there, you might be very surprised....
Who here has put their eye on the new 661 combustion chamber, and measured the width and taper of the squish band?
I think if you take a hard look at what Stihl did there, you might be very surprised....
Trx250r180
Saw polisher
Does this thread have anything in common like i was asking you about last week in the pm ?
drf255
BAD CAD
Youre the mopar guy,
But alot of that 60's tech was to fit bigger valves and centrally locate the plug. They even made a 4 valve dual plug hemi at one point. Chevy had a 302 hemi as well.
It's my understanding that the new "HEMI" from chrysler is not a true hemispherical combustion chambered head. It's more of a bastardized pent roof/hemi chamber with some squish.
A marketing ploy.
Someone should be able to post a pic. I could be off here, but thats what ive been told by owners and mechanics.
Take a looksie.
